Apparatus, system, and method for improved calibration and measurement of differential devices

ABSTRACT

An apparatus, system and method for facilitating the calibration and measurement of differential devices by a variety of laboratory fixtures. An adapter is used to transition the connection of standard coaxial interfaces to a hermaphroditic differential interface while compensating for discontinuities on impedance at the connection. The adapter includes a transition region with conductors and shield dimensions that compensate for the discontinuity in impedance. The adapter has pin and socket inputs so that a mated pair of the adapters provides an insertable device for test and measurement on a four port vector network analyzer.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/689,589 (pending) filed Oct. 22, 2003.

FIELD OF THE INVENTION

The present invention is directed generally to adapters and connectorsused to facilitate differential S-parameter calibration and measurementof differential devices by a variety of laboratory fixtures.

BACKGROUND OF THE INVENTION

The design of the coaxial interfaces and coaxial transmission lines is awell understood art. For a given impedance, the geometry of adifferential transmission line is readily calculated using a twodimensional field solution algorithm. When making measurements ofdifferential devices, the measurement instruments typically have 50 Ohmcoaxial interfaces appropriate for the frequency band of interest.

However, most calibration procedures make measurements at single-endedports, whether using PCB probes or some other measurement device. Whenthe differential device is measured, the conversion from single-ended todifferential propagation takes place at the transition from thecalibration ports to the differential device under test (DUT). Anyimbalance or mode conversion that takes place at this transition becomesa part of the measurement itself. Thus, there are advantages of makingthe transition within an adapter or connector to present a differentialinterface to the DUT.

Moreover, in order to more accurately measure differential devices, itis desirable to convert from the coaxial instrument ports to thedifferential interface geometry of the DUT before attachment to the DUTin a way that includes the conversion as part of the calibrationprocedure.

Thus, it would be advantageous to have an adapter or connector thatprovides a connection between coaxial to differential environments andcompensate for discontinuities in impedance at the transition.

SUMMARY OF THE INVENTION

With the foregoing in mind, the present invention provides an apparatus,system and method for converting single-ended signals of a first devicehaving coaxial interfaces to a differential signal of a second devicehaving a differential interface while maintaining uniform differentialimpedance. Specifically, the present invention utilizes an adapter withat least two coaxial interfaces for coupling to coaxial devices;conductors for transmitting signals through the adapter; a transitionregion for providing a connection between coaxial and differentialtransmission environments and for maintaining a uniform differentialimpedance at the connection; and a differential interface for couplingto a differential device.

In one aspect of the invention, the coaxial interfaces are standardprecision coaxial interfaces, such as a 3.5 millimeter coaxialinterface, that converge at an angle to form at least one differentialline inside the adapter. The angle of convergence, which is the anglebetween the axis of the coaxial interface and the axis of thedifferential device, of the coaxial interfaces may vary fromapproximately 0 to 90-degrees, with a preferred angle of 10-degrees.Additionally, the transition region includes conductors and shielddimensions that compensate for the differential impedance discontinuityat the connection between the coaxial and differential environments, andprovides variations in the diameter and dimensions of the conductors.The transition region further includes a dielectric support structure ator near said connection between the coaxial and differentialtransmission environments. In a preferred embodiment, the differentialinterface has a 4 millimeter outer diameter with hermaphroditic pin andsocket signal contact and means for mating to other devices formeasurement with a 4-port vector network analyzer or a variety of otherinstrumentation such as a signal generator, oscilloscope or a 2-porttime domain reflectometer.

In another aspect of the invention, the conversion from single-endedcoaxial to differential propagation as well as compensation fordifferential impedance discontinuity is achieved prior to connection toa DUT. More specifically, the method for converting from single-endedcoaxial propagation to differential propagation involves coupling atleast two coaxial transmission lines from a first device to at least twocoaxial interfaces of an adapter. The adapter provides a connectionbetween coaxial and differential transmission environments andcompensates for differential impedance discontinuity within a transitionregion. The adapter is then coupled at a differential interface to adifferential device. To this end, the discontinuities in impedance arecompensated for prior to connection to the differential device.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures best illustrate the details of the apparatus,system and method of the present invention. Like reference numbers anddesignations in these figures refer to like elements.

FIG. 1 is a system drawing in accordance with an embodiment of thepresent invention.

FIG. 2 is a detailed drawing of the differential adapter in accordancewith an embodiment of the present invention.

FIG. 3 is a detailed drawing of the transition region of thedifferential adapter in accordance with an embodiment of the presentinvention.

FIG. 4 a is a detailed drawing of the differential interface of thedifferential adapter in accordance with an embodiment of the presentinvention.

FIG. 4 b provides a view of the differential interface end of thedifferential adapter.

FIGS. 5-7 show graphs of S-parameter data collected in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a system diagram in accordance with an embodiment of thepresent invention. In FIG. 1, a four-port vector network analyzer (VNA)1 is connected to the first of two differential adapters 7 via twocoaxial interconnect cables 3. While a four-port VNA 1 is shown in FIG.1, a scenario in which the system includes instrumentation, such as asignal generator, oscilloscope or time domain reflectometer havingsingle-ended ports, for interfacing with a differential device is alsocontemplated. One end of the coaxial interconnect cables 3 is connectedto the VNA at respective coaxial outputs 2 while the other end of thecoaxial interconnect cable 3 is connected to the differential adapter 7on one end at respective coaxial interfaces 5 using coaxial connectors 4on the ends of the cable 3. For simplification, the coaxial interfaces 5of the adapter 7 appear parallel. However, the coaxial interfaces 5 mayconverge at variable angles with respect to each other. The coaxialinterfaces 5 of differential adapter 7 accommodate connection ofstandard precision coaxial control interfaces, such as those with a 1.0millimeter, 1.85 millimeter, 2.4 millimeter, 3.5 millimeter or 7millimeter shield inner diameter or any other calibratable, compatiblesubstitutes to a hermaphroditic differential interface. Furthermore, oneof ordinary skill in the art will recognize that other compatible,non-precision interfaces that are either coaxial or differential innature may be used to provide a suitable product.

One end of the differential adapter 7 also includes a differentialinterface 9 that is connected to one end of the DUT 11 via adifferential connector 10. In a preferred embodiment, the differentialinterface 9 of the differential adapter 7 has a 4 millimeter outerdiameter with hermaphroditic pin/socket signal contacts (not shown). Oneof ordinary skill in the art will recognize that differential adaptershaving larger or smaller outer diameters may be used. For mechanicallycoupling differential interfaces, 7 millimeter hermaphroditic or othersuitable couplings (not shown) can also be used. The differentialinterface 9 also incorporates alignment pins (not shown in FIG. 1) toprovide registration of the signal contact prior to mating with anotherdevice. The other end of the DUT 11 is attached to one end of the seconddifferential adapter 7. The second adapter is then connected to thecoaxial inputs 12 of the VNA 1 via a second set of coaxial interconnectcables 3. This system setup can be used to perform s-parametercalibration and measurements of differential devices. Using two coaxialto differential adapters 7 of the present invention, calibration can beperformed without the DUT 11 being attached in the circuit. A detailedexplanation of the characteristics of the differential adapter that makethis possible will be presented in the figures that follow.

FIG. 2 provides a more detailed drawing of the differential adapter 7 inaccordance with an embodiment of the present invention. Specifically,this view shows a complete differential adapter 7 with two coaxialinterfaces or ports 5 consisting of inner signal conductors 15 and outerconducting shields 17 separated by air dielectric 14. Each innerconductor 15 is supported by a dielectric bead 21. The coaxialinterfaces 5 incorporate a coupling mechanism, either a coupling nut 23or a coupling thread 25. The center section of differential adapter 7,shown in detail in FIG. 3, illustrates the transition region 16 of thedifferential adapter 7. This portion of the differential adapter isreferred to as the transition region because it provides the transitionfrom coaxial to differential transmission lines. The differentialinterface 9 on the left is shown in detail in FIG. 4 a.

FIG. 3 depicts the bifurcated transmission line transition from dualcoaxial lines, having independent signal conductors 15, to a singledifferential transmission line having electrically coupled signalconductors 29. Each coaxial line must be dimensioned such that they haveequal impedance values that combine to produce a differential impedanceequal to that of the differential impedance of the single differentialtransmission line. The transition region 16 where the coaxial conductorsand differential transmission lines meet has signal conductor and shieldconductor 31 dimensions fabricated so that the differential impedancediscontinuity caused by the juncture is compensated to maintain auniform differential impedance. There is an air space 35 between thedifferential conductors 29 and the shield conductor 31. Note that withinthe transition region 16, the independent signal conductors 15 from thecoaxial lines and the differential signal conductors 29 may beconstructed from a single conductor. Alternatively, for the ease ofmanufacturing, within the transition region 16, the independent signalconductors 15 may be joined to differential signal conductors 29 bycouplers well known in the art such as a pin and socket or by splicing.In other embodiments, a dielectric supporting structure 33 forsupporting the signal conductors is incorporated at or near the junctureand/or a splice in the signal conductors to facilitate fabrication ofthe adapter. The impedance discontinuities caused by such alternativeconstructions are accommodated by additional dimensional modificationsof the signal conductors or the outer conducting shield to provide theappropriate impedance compensation.

FIG. 4 a provides a more detailed view of the differential interface 9to other differential DUTs. The two differential signal conductors 29are supported relative to the conducting shield 31 by a dielectricsupport structure 33. The two differential signal conductors 29 havebeen configured here such that one presents a female jack 37 for joiningto a mating connector's conductor. The other conductor has a male plug39. At the interface surface, an alignment pin 18 and alignment hole 43provide the capability to correctly align the conductors 29 to easejoining to a mating connector and to maintain the differential impedancethrough the interface 10. The alignment pin 18 serves two purposes inaddition to ensuring correct alignment. First, the length of thealignment pin 18 is longer than the male plug 39, providing a level ofprotection to the latter. Second, when two adapters are mated througheach adapter's differential interface 9, proper mating of each specificsignal conductor is achieved. If a male and male differential interfaceis employed rather than a hermaphroditic interface, two alignment pinsmay be provided on the male and male interface with two alignment holesprovided on the mated female and female interface.

Between the conductors 29 within the differential interface 9 is an airspace 45. This air space 45 allows for a particular type of calibrationthat uses a length of “unsupported” transmission line, often referred toas an “air line”, which is essentially two differential interfacessituated back to back with a length of transmission line between thedifferential interfaces. The signal conductors 29 are supported by thepin or male plug 39 and socket of female jack 37 connections at eachend.

The relative dimensions and spacing of the signal conductors 29 withinthe differential interface 9 will now be described. The signalconductors 29 have a diameter that is approximately one-quarter of theinner diameter of the shield conductor 31 at the interface. Furthermore,each of the signal conductor's 29 center axis is spaced approximatelyequidistant between the inner surface of the shield conductor 31 at theinterface and the center axis of the shield conductor 31. Thus, if theshield conductor 31 has an inner diameter of “4A”, then the signalconductors 29 are each placed on a line passing through the axis of theshield conductor 31 and each signal conductor axis is a distance “A”away from the shield conductor 31 inner surface and “2A” away from eachother. These dimensions along with the conductor placement provide themost stable differential impedance with respect to slight variations ofconductor placement.

While the differential interface 9 may be hermaphroditic, they may notbe, depending on the application. The differential interface 9 mayprovide either both sexes of contacts or two contacts of the same sex.While a hermaphroditic interface may be preferable, in some situations,contacts of the same sex may be useful as well. Use of a hermaphroditicinterface does allow mating of any two connectors without regard for thesex of the connector.

FIG. 4 b provides a view of the differential interface 9 end of thedifferential adapter 7. From this perspective, one can recognize thatthe body of the differential adapter 7 is cylindrical in shape.

FIGS. 5, 6 and 7 represent various s-parameter data for the differentialadapter. FIG. 5 shows a low level of reflection in differential modepassing through a mated pair of the differential adapters of the presentinvention. The data depicted in FIG. 6 indicate minimal mode conversionby reflection at the input coaxial port of a mated pair of differentialadapters. FIG. 7 shows that there is minimal mode conversion duringtransmission through a mated adapter pair.

Although illustrative embodiments have been described herein in detail,its should be noted and understood that the descriptions and drawingshave been provided for purposes of illustration only and that othervariations both in form and detail can be added thereupon withoutdeparting from the spirit and scope of the invention. The terms andexpressions have been used as terms of description and not terms oflimitation. There is no limitation to use the terms or expressions toexclude any equivalents of features shown and described or portionsthereof.

1. An adapter for converting single-ended coaxial signals todifferential signals, comprising: at least two coaxial interfaces forcoupling to a coaxial device; a first signal conductor for transmittingsignals through the adapter; a second signal conductor for transmittingsignals through the adapter; a shield conductor; a transition region forproviding a transition between coaxial and differential transmissionenvironments and maintaining a uniform differential impedance throughthe transition; and a differential interface for coupling to adifferential device.
 2. The adapter of claim 1, wherein the transitionregion includes the first signal conductor, the second signal conductorand the shield conductor, having dimensions providing uniformdifferential impedance.
 3. The adapter of claim 1, wherein thetransition region includes a coupling device for connecting a firstportion of the first signal conductor to a second portion of the firstsignal conductor and connecting a first portion of the second signalconductor to a second portion of the second signal conductor.
 4. Theadapter of claim 1, wherein in the transition region, a first portion ofthe first signal conductor is spliced together with a second portion ofthe first signal conductor and a first portion of the second signalconductor is spliced together with a second portion of the second signalconductor.
 5. The adapter of claim 1, wherein the differential interfaceincludes an air space between the first conductor and the secondconductor.
 6. The adapter of claim 1, wherein the at least two coaxialinterfaces are standard precision coaxial interfaces.
 7. The adapter ofclaim 6, wherein the diameter of the at least two coaxial interfaces isselected from the group consisting of 1.0 millimeter, 1.85 millimeters,2.4 millimeters, 3.5 millimeters or 7 millimeters.
 8. The adapter ofclaim 1, wherein a center axis of each of the at least two coaxialinterfaces is situated at an angle between 0 and 90 degrees from thecenter axis of the differential interface.
 9. The adapter of claim 1,wherein the differential interface further comprises a mating member formating to another device for measurement with a 4-port vector networkanalyzer.
 10. The adapter of claim 1, wherein said transition regionfurther includes a dielectric support structure at or near thetransition between the coaxial and differential transmissionenvironments.
 11. The adapter of claim 1, wherein the diameter anddimensions of the first conductor and second conductor vary within thetransition region.
 12. The adapter of claim 1, further comprisingdielectric beads for supporting the first signal conductor and thesecond signal conductor.
 13. The adapter of claim 1, wherein a centeraxis of each of the at least two coaxial interfaces is situated at anangle of 10-degrees from the center axis of the differential interface.14. An interface apparatus for connecting a differential device to anadapter for converting single-ended coaxial signals to differentialsignals, comprising: a first differential signal conductor; a seconddifferential signal conductor; and a shield conductor, wherein thediameter of the shield conductor is substantially equal to four timesthe diameter of the first differential signal conductor.
 15. Theinterface apparatus of claim 14, wherein a center axis of the firstdifferential signal conductor and a center axis of the seconddifferential signal conductor are each situated at a substantially equaldistance between an inner surface of the shield conductor and a centeraxis of the shield conductor.
 16. The interface apparatus of claim 14,wherein the first differential signal conductor is situated at adistance substantially equal to half of the diameter of the shieldconductor from the second differential signal conductor.
 17. Theinterface apparatus of claim 14, wherein the first conductor and secondconductor are each situated at a distance substantially equal to onequarter of the diameter of the shield conductor from an inner surface ofthe shield conductor.
 18. The interface apparatus of claim 14, furthercomprising contacts that include a male contact and a female contact.19. The interface apparatus of claim 14, further comprising contactsthat are the same sex.
 20. The interface apparatus of claim 14, furthercomprising hermaphroditic pin and socket contacts allowing mating with adifferential device regardless of the sex of the contacts of thedifferential device.
 21. The interface apparatus of claim 14, furthercomprising an alignment hole and an alignment pin providing correctalignment of a mated pair of adapters.
 22. The interface apparatus ofclaim 14, wherein the alignment pin is longer than a connecting plug ofthe differential interface to provide protection for the connectingplug.
 23. The interface apparatus of claim 14, wherein the interfaceapparatus has a 4 millimeter outer diameter.
 24. An apparatus forconverting a single-ended signal of a first device having a coaxialinterface to a differential signal of a second device having adifferential interface while maintaining uniform differential impedanceat a connection between coaxial and differential environments, whereinsaid first device is a testing device and said second device is a deviceunder test.
 25. A system for converting from single-ended coaxialsignals to differential signals, comprising: a single-ended coaxialdevice; an adapter with at least two coaxial interfaces for coupling tothe single-ended coaxial device; at least two signal conductors fortransmitting signals through the adapter; a transition region within theadapter for providing a transition between coaxial and differentialtransmission environments and maintaining a uniform differentialimpedance through the transition; and a differential interface on oneend of the adapter for coupling to a differential device.
 26. The systemof claim 25, wherein the transition region includes the at least twosignal conductors and a shield conductor, the dimensions of whichprovide the uniform differential impedance.
 27. The system of claim 25,wherein the transition region includes a coupling device for connectinga first portion of one of the at least two signal conductors to a secondportion of the one of the at least two signal conductors and connectinga first portion of the second of the at least two signal conductors to asecond portion of the second of the at least two signal conductors. 28.The system of claim 25, wherein the transition region includes a firstportion of one of the at least two signal conductors spliced togetherwith a second portion of the one of the at least two signal conductorsand a first portion of a second of the at least two signal conductorsspliced together with a second portion of the second of the at least twosignal conductors.
 29. The system of claim 25, wherein the differentialinterface includes an air space between the first conductor and thesecond conductor.
 30. The system of claim 25, wherein the at least twocoaxial interfaces are standard precision coaxial interfaces.
 31. Thesystem of claim 30, wherein the diameter of the at least two coaxialinterfaces is selected from the group consisting of 1.0 millimeter, 1.85millimeters, 2.4 millimeters, 3.5 millimeters or 7 millimeters.
 32. Thesystem of claim 25, wherein a center axis of each of the at least twocoaxial interfaces is situated at an angle between 0 and 90 degrees fromthe center axis of the differential interface.
 33. The system of claim25, wherein the differential interface further comprises a mating memberfor mating to another device for measurement with a 4-port vectornetwork analyzer.
 34. The system of claim 25, wherein said transitionregion further includes a dielectric support structure at or near thetransition between the coaxial and differential transmissionenvironments.
 35. The system of claim 25, wherein the diameter anddimensions of the first conductor and second conductor vary within thetransition region.
 36. The system of claim 25, further comprisingdielectric beads for supporting the first signal conductor and thesecond signal conductor.
 37. The system of claim 25, wherein thedifferential interface has a 4 millimeter outer diameter withhermaphroditic pin and socket signal contacts.
 38. A method forconverting from single-ended coaxial signals to differential signals,comprising: coupling at least two coaxial transmission lines from afirst device to at least two coaxial interfaces of an adapter; providinga connection within said adapter between coaxial and differentialtransmission environments; compensating for differential impedancediscontinuity at the connection using a transition region of saidadapter; and coupling a differential interface of said adapter to adifferential device.
 39. The method of claim 38, further comprisingvarying the dimensions of signal conductors and a shield conductorwithin said transition region to compensate for differential impedancediscontinuity at the connection.
 40. The method of claim 38, furthercomprising converging the at least two coaxial interfaces at a 10 degreeangle to form at least one differential line inside the adapter.
 41. Themethod of claim 38, further comprising mating the adapter at thedifferential interface with another device for measurement with a 4-portvector network analyzer.
 42. The method of claim 38, further comprisingsupporting signal conductors at the transition region at or near saidconnection using a dielectric support structure.
 43. The method of claim38, further comprising supporting signal conductors in the coaxialinterfaces using dielectric beads.
 44. A method for converting fromsingle-ended coaxial signals to differential signals by providing aconnection between coaxial and differential transmission environmentsthat compensates for differential impedance discontinuity at aconnection between coaxial and differential environments, wherein saidcompensation is achieved prior to connection to a device under test. 45.An insertable test device, comprising: a first adapter having a firstdifferential interface; and a second adapter having a seconddifferential interface, wherein each of the first adapter and secondadapter includes at least two coaxial interfaces for coupling to acoaxial device, a first signal conductor for transmitting signalsthrough the adapter, a second signal conductor for transmitting signalsthrough the adapter, a shield conductor, and a transition region forproviding a transition between coaxial and differential transmissionenvironments and maintaining a uniform differential impedance throughthe transition; and the first adapter and the second adapter are matedat the first differential interface and the second differentialinterface.